U.S. patent number 5,288,304 [Application Number 08/052,423] was granted by the patent office on 1994-02-22 for composite carbon fluid separation membranes.
This patent grant is currently assigned to The University of Texas System. Invention is credited to Cheryl W. Jones, William J. Koros.
United States Patent |
5,288,304 |
Koros , et al. |
February 22, 1994 |
Composite carbon fluid separation membranes
Abstract
The invention provides carbon membranes for use in fluid
separation processes, particularly gas separations, which are
treated with a coating that provides a protective barrier which
significantly limits permeation of water vapor or other impurities
such as hydrocarbons without significantly inhibiting permeation of
the faster fluid component or lowering selectivity. The composite
membranes retain good fluid separation properties and are resistant
to the adverse effects on membrane performance commonly observed in
environments having high humidity. The coating is preferably an
amorphous polymer of perfluoro-2,2-dimethyl-1,3-dioxole. The
membranes can be of a varied configuration: sheet form, hollow
fiber, asymmetrical membranes and the like.
Inventors: |
Koros; William J. (Austin,
TX), Jones; Cheryl W. (Austin, TX) |
Assignee: |
The University of Texas System
(Austin, TX)
|
Family
ID: |
21977523 |
Appl.
No.: |
08/052,423 |
Filed: |
March 30, 1993 |
Current U.S.
Class: |
95/45; 95/54;
96/10; 96/13 |
Current CPC
Class: |
B01D
53/228 (20130101); B01D 67/0067 (20130101); B01D
67/0088 (20130101); B01D 69/02 (20130101); B01D
69/12 (20130101); B01D 71/021 (20130101); B01D
71/44 (20130101); B01D 69/08 (20130101); B01D
2325/32 (20130101); B01D 2325/022 (20130101) |
Current International
Class: |
B01D
71/44 (20060101); B01D 69/12 (20060101); B01D
53/22 (20060101); B01D 67/00 (20060101); B01D
71/00 (20060101); B01D 69/00 (20060101); B01D
71/02 (20060101); B01D 053/22 () |
Field of
Search: |
;55/16,158,524 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
0337499 |
|
Oct 1989 |
|
EP |
|
63-264101 |
|
Nov 1988 |
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JP |
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4198918 |
|
Jul 1990 |
|
JP |
|
2207666A |
|
Feb 1989 |
|
GB |
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Levitt; Cary A.
Claims
We claim:
1. A membrane for the separation of fluids comprising a carbon
membrane coated with a coating that reduces the partial pressure of
water or other condensible agents or impurities relative to the
other permeable components and having resistance to the permeation
of water or other impurities without causing significant losses to
the selectivity or productivity of the membrane.
2. A membrane of claim 1 wherein the coating is selected from the
group of poly( 4-methyl-1-pentene), polyethylene and
silicon-containing polymers.
3. A membrane of claim 2 wherein the coating is polysiloxane or
poly(1-trimethyl silyl) propyne.
4. A membrane of claim 1, wherein the carbon membrane is in sheet
form having a thickness from about 1 .mu.m to about 300 .mu.m.
5. A membrane of claim 1, wherein the carbon membrane is in the
form of hollow fibers, having a diameter from about 50 .mu.m to
about 1000 .mu.m diameter and with a wall thickness of about 10
.mu.m to about 300 .mu.m.
6. A membrane of claim 1, wherein the carbon membrane is of
asymmetrical construction.
7. A membrane of claim 1, wherein the carbon membrane is the
product of pyrolysis of a carbon-containing precursor at at
temperature of about 250.degree. C. to about 2500.degree. C.
8. A membrane for the separation of fluids comprising a carbon
membrane coated with a coating of a polymer having an aliphatic
ring structure containing fluorine.
9. A membrane of claim 8 wherein the polymer comprises the group of
repeating units represented by the following general formula:
##STR2## where n is an integer of 1 or 2, or copolymers
thereof.
10. A membrane of claim 8 wherein the polymer is an amorphous
polymer of perfluoro-2,2,-dimethyl-1,3-dioxole.
11. The membrane of claim 10 in which the polymer is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxole.
12. The membrane of claim 11 in which the polymer is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and a complementary amount of at
least one monomer selected from the group consisting of
tetrafluoroethylene, perfluoromethyl vinyl ether, vinylidene
fluoride and chlorotrifluoroethylene.
13. The membrane of claim 10 in which the polymer is a homopolymer
of perfluoro-2,2-dimethyl-1,3-dioxole.
14. The membrane of claim 10 in which the polymer is a dipolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and a complementary amount of
tetrafluoroethylene.
15. The membrane of claim 14 in which the polymer is a dipolymer
containing 65-99 mole % of perfluoro-2,2-dimethyl-1,3-dioxole and
having a glass transition temperature of at least 140.degree.
C.
16. A membrane of claim 8, wherein the carbon membrane is in sheet
form having a thickness from about 1 .mu.m to about 300 .mu.m.
17. A membrane of claim 8, wherein the carbon membrane is in the
form of hollow fibers, having a diameter from about 50 .mu.m to
about 1000 .mu.m diameter and with a wall thickness of about 10
.mu.m to about 300 .mu.m.
18. A membrane of claim 8, wherein the carbon membrane is of
asymmetrical construction.
19. A membrane of claim 8, wherein the carbon membrane is the
product of pyrolysis of a carbon-containing precursor at
temperature of about 250.degree. C. to about 2500.degree. C.
20. A separation module of the shell and tube type comprising a
plurality of hollow membranes as described in claim 5 or 17.
21. A process for separating fluids comprising contacting a mixture
of fluids with the first side of the membrane described in any one
of claims 1-19 in a manner to cause a portion of the mixture to
pass through the membrane to a permeate side, the resulting mixture
on the permeate side being enriched in one or more component over
that of the mixture on the first side.
Description
FIELD OF THE INVENTION
The invention relates to novel carbon fluid separation membranes
and a process for the separation of fluids, particularly gases. In
particular, the invention provides composite membranes for use in
fluid separations, which are treated with a coating that provides a
protective barrier which significantly reduces the adverse effects
on the performance of the membrane caused by high humidity or other
impurities in the fluid to be treated.
BACKGROUND OF THE INVENTION
The use of membranes for separation processes is well known.
Certain carbon membranes are particularly useful for the separation
of fluids, especially gases such as oxygen and nitrogen.
The membranes may be fabricated in various geometrical
configurations, such as sheet formed membranes and hollow fibers.
The membranes may be symmetrical, asymmetrical, single-component or
composite.
Carbon membranes have superior selectivities and productivities for
many separations. However, a major problem with these membranes has
been their vulnerability to the effects of water vapor and other
condensible agents and impurities such as oils or other hydrocarbon
compounds. For example, humidity levels well below 100% relative
humidity are sufficient to significantly impair the performance of
the carbon membrane. Small amounts of oil or other hydrocarbons can
also significantly impair the performance of the membrane.
In order to reduce the humidity of the fluid to be permeated, the
fluid may be treated with dehumidifying agents. This typically
involves the use of large, expensive equipment. Such equipment is
also prone to failure. In addition, other condensible agents and
impurities may be removed from the fluid to be permeated by various
filtration, separation or extraction techniques. These measures may
also involve the use of large, expensive equipment and are often
not successful.
It is known to prepare composite membranes and/or post treat
membranes with materials that seal or heal defects or improve the
stability of the membrane. For example, U.S. Pat. Nos. 3,616,607
and 3,775,303 exemplify gas separation membranes having
superimposed membranes on a porous support.
U.S. Pat. No. 4,230,463 deals broadly with the post treatment of
fluid separation membranes. It describes a wide variety of
membranes for liquid and gas separations, particularly a
multicomponent membrane where the separation properties of the
membrane are principally determined by the porous separation
membrane as opposed to the material of the coating. The coating
cures defects in the surface of the membrane. U.S. Pat. No.
4,767,422 also discloses a method of posttreating composite
membranes to cure defects in the thin separation layer. U.S. Pat.
No. 4,728,345 describes a multicomponent membrane for gas
separation having a polyphosphazene coating in occluding contact
with a porous separation membrane for the purpose of improving
stability of the membrane when exposed to aromatic and aliphatic
hydrocarbons contained in a gaseous mixture.
EPO Patent Application 0,337,499 discloses a gas separation
membrane with a covering layer formed from a selective film. The
covering layer is made from a polymer having a critical surface
tension not larger than 30 dynes/cm, such as
poly-4-methylpentene-1, fluorinated alkyl methacrylate and
polymethyl fluorinated alkyl siloxane.
U.S. Pat. No. Re. 33,273 describes a method of improving the
characteristics of separatory membranes by the deposition of a
fluorinated amphiphilic compound in an oriented layer on the
surface of the membrane so as to increase membrane selectivity and
counteract membrane surface properties leading to fouling caused by
colloidal materials.
The prior art references do not, however, teach a polymeric
membrane treatment for reducing the adverse effects of impurities
on the performance of carbon membranes. A carbon membrane is,
therefore, needed with good permeation properties and significant
resistance to the effects of water vapor and other condensible
agents and impurities. The inventive fluid separation membrane is a
composite carbon membrane which retains high selectivities for
fluid separations and is significantly more resistant to the
adverse affects commonly observed in environments having high
humidity or other impurities.
SUMMARY OF THE INVENTION
This invention relates to novel fluid separation membranes and a
process for the separation of fluids. A carbon membrane is coated
with a thin layer of polymeric material that offers resistance to
water vapor permeation. Preferably, the coating does not
significantly inhibit permeation of the fluids to be separated,
does not significantly lower the membrane's selectivity, is
chemically resistant to the fluids to be separated and does not
decompose at high temperatures. Suitable coatings include
hydrocarbon polymers such as poly(4-methyl-1-pentene), polymers
having an aliphatic ring structure containing fluorine, preferably
amorphous polymers of perfluoro-2,2-dimethyl-1,3-dioxole and
silicone-containing polymers such as polysiloxanes and
poly(1-trimethylsilyl) propyne. The resulting carbon membranes
retain high selectivities for fluid separations and are also
significantly more resistant to the adverse affects observed in
environments having high humidity or having other condensible
agents or impurities such as oil or other hydrocarbons. These
composite membranes are widely useful in producing nitrogen
enriched air for applications such as nitrogen blanketing for food,
pharmaceutical uses and fuel storage applications. These composite
membranes are also effective in the separation of other gases
including carbon dioxide/methane, hydrogen/nitrogen and
hydrogen/methane.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B and 1C are schematic drawings which show the test
module used to characterize the properties of the carbon
membranes.
FIG. 2 is a schematic drawing which shows a diagram of the membrane
test system.
DETAILED DESCRIPTION OF THE INVENTION
Fluid separation membranes are well known in the art. Preferably,
the membranes have pores of a predetermined narrow range of size.
The size can be varied at will within certain limits, and for the
separation of gaseous mixtures membranes advantageously have a pore
size of definite value, which can be varied in membranes for
various purposes from about 2.5 Angstrom to about 10 Angstrom,
preferably 3 to 5 Angstrom.
Membranes of flat configuration (in sheet form) are generally of a
thickness of from about 1 .mu.m to about 50 .mu.m, although for
different purposes different thicknesses can be used. With
asymmetrical flat membranes the thickness of the effective
separating layer of the membrane can be even thinner than 1 .mu.m.
When the membrane is used in the form of hollow fibers, the
diameter will generally vary between 5 .mu.m and 1 mm, preferably
50 to 1000 .mu.m with a wall thickness of from about 1 .mu.m to
about 300 .mu.m, preferably about 10 .mu.m-100 .mu.m according to
the diameter.
For purposes of this invention, the fluid separation membranes are
carbon membranes.
U.S. Pat. No. 4,685,940 teaches carbon membranes for use in
separation processes. Carbon membranes have a predetermined pore
size and function as molecular sieves. Carbon membranes function
well even at elevated temperatures. Carbon membranes used in the
present invention are produced by the controlled pyrolysis of a
suitable polymeric material under conditions which retain the basic
integrity of the original geometry. Suitable materials include
polyimides, polyamides, cellulose and derivatives thereof,
thermosetting polymers, acrylics, pitch-tar mesophase, and the
like. These materials are not limiting, as other materials may be
useful for fabricating carbon membranes. Selection of the polymeric
material for the carbon membrane for fluid separations may be made
on the basis of the heat resistance, solvent resistance, and
mechanical strength of the porous separation membrane, as well as
other factors dictated by the operating conditions for selective
permeation.
The pyrolysis can be generally effected in a wide range of
temperatures, between the decomposition temperature of the
carbonaceous material and the graphitization temperature (about
3000.degree. C.). Generally, pyrolysis will be effected in the
range of from 250.degree. C. to 2500.degree. C., a preferred range
being about 450.degree. C. to about 800.degree. C.
The carbon membranes contain pores larger than the ultramicropores
required for the molecular seiving process. These larger pores
connect the ultramicropores that perform the molecular sieving
process and allow for high productivities in the dry membrane
state. Generally, the higher the final temperature used for the
pyrolysis of the polymer, the smaller are the pores of the product,
and thus the smaller the molecules which could permeate through
such membranes.
One of the primary advantages of carbon membranes is their ability
to effect gaseous separations at high temperatures. The separation
can be effected at any desired temperature, up to temperatures
where carbon membranes begin to deteriorate. For nonoxidizing
gases, this temperature may be as high as about 1000.degree. C.
The pyrolysis of suitable precursors, generally under conditions
conventionally used for the production of carbon fibers, results in
a product which has a certain microporosity of molecular dimensions
which is responsible for the molecular sieve properties of the
carbons.
During the pyrolysis process, the heating is preferably effected
under an inert atmosphere such as nitrogen or noble gas which aids
in controlling oxidation. Controlled oxidation results in a pore
opening, and thus predetermined pore-size ranges can be obtained,
suitable for the intended separation process. Suitable oxidizing
agents include oxygen, steam, carbon dioxide, nitrogen oxides and
chlorine oxides, solutions of nitric acid, sulfuric acid, chromic
acid and peroxide solutions. After oxidation treatment the membrane
should be degassed at elevated temperatures. Asymmetrical membranes
can be prepared by the controlled pyrolysis of conventional
asymmetrical organic membranes having the required structure. Such
membranes can also be produced by the deposition of a thin
permselective carbon layer on a porous support by methods known in
the art.
For the intended use, it is advantageous to obtain fluid separation
membranes having pore size and a pore size distribution that
effectively separate specific mixtures of fluids. Generally, a pore
size of 3-10 Angstrom is suitable and 3-5 Angstroms is preferable
for gas separations.
By depositing a layer or coating of certain protective or barrier
materials on surface of the polymer or carbon separation membrane,
the adverse effects of humidity or other impurities on the
performance of the membranes may be minimized. Suitable materials
must offer some resistance to the permeability of water vapor or
other condensible agents or impurities while not prohibitively
inhibiting the permeability of the fluids to be separated.
Preferably, the protective or barrier coating is easily solubilized
so that it may be coated on the surface of the membrane. In
addition, the protective or barrier coating is preferably
chemically inert and resistant to decomposition at elevated
temperatures.
Suitable coatings include polymers having an aliphatic ring
structure containing fluorine, for example an amorphous polymer of
perfluoro-2,2-dimethyl-1,3-dioxole. In embodiments, the polymer is
a homopolymer of perfluoro-2,2-dimethyl-1,3-dioxole. In other
embodiments, the polymer is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxole, including copolymers having a
complementary amount of at least one monomer selected from the
group consisting of tetrafluoroethylene, perfluoromethyl vinyl
ether, vinylidene fluoride and chlorotrifluoroethylene. In
preferred embodiments, the polymer is a dipolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and a complementary amount of
tetrafluoroethylene, especially such a polymer containing 65-99
mole % of perfluoro-2,2-dimethyl-1,3-dioxole. The amorphous polymer
preferably has a glass transition temperature of at least
140.degree. C., and more preferably at least 180.degree. C. Glass
transition temperature (T.sub.g) is known in the art and is the
temperature at which the polymer changes from a brittle, vitreous
or glassy state to a rubbery or plastic state. Examples of
dipolymers are described in further detail in U.S. Pat. No.
4,754,009 and U.S. Pat. No. 4,935,477, both of E. N. Squire. The
polymer may, for example, be an amorphous copolymer of
perfluoro(2,2-dimethyl-1,3-dioxole) with a complementary amount of
at least one other comonomer, said copolymer being selected from
the group consisting of dipolymers with perfluoro(butenyl vinyl
ether) and terpolymers with perfluoro(butenyl vinyl ether) and with
a third comonomer, wherein the third comonomer can be (a) a
perhaloolefin in which halogen is fluorine or chlorine, or (b) a
perfluoro(alkyl vinyl ether); the amount of the third comonomer,
when present, preferably being at most 40 mole % of the total
composition. Polymerization is performed by methods known in the
art.
Other suitable polymers having an aliphatic ring structure
containing fluorine are described in U.S. Pat. No. 4,897,457 of
Nakamura et al. and Japanese Published Patent Application Kokai
4-198918 of Nakayama et al; e.g., a fluorine-containing
thermoplastic resinous polymer consisting of a group of repeating
units to be represented by the following general formula: ##STR1##
(where: n is an integer of 1 or 2); and copolymers thereof.
The glass transition temperature of the amorphous polymer will vary
with the actual polymer of the membrane, especially the amount of
tetrafluoroethylene or other comonomer that may be present.
Examples of T.sub.g are shown in FIG. 1 of the aforementioned U.S.
Pat. No. 4,754,009 of E.N. Squire as ranging from about 260.degree.
C. for dipolymers with tetrafluoroethylene having low amounts of
tetrafluoroethylene comonomer down to less than 100.degree. C. for
the dipolymers containing at least 60 mole % of
tetrafluoroethylene.
In preferred embodiments of the membranes and methods of the
present invention, the polymer is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxole, especially a copolymer having a
complementary amount of at least one monomer selected from the
group of tetrafluoroethylene, perfluoromethyl vinyl ether,
vinylidene fluoride and chlorotrifluoroethylene.
In other embodiments, the polymer is a homopolymer of
perfluoro-2,2-dimethyl-1,3-dioxole.
In further embodiment, the polymer is a dipolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and a complementary amount of
tetrafluoroethylene.
Suitable coatings also include poly(4-methyl-1-pentene), and
silicon-containing polymers such as polysiloxanes and
poly(1-trimethyl silyl) propyne. The preferred coating is
TEFLON.RTM. AF (commerically available from E. I. du Pont de
Nemours and Company) which is a dipolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and tetrafluoroethylene.
While any suitable method can be employed, the method by which the
coating is applied can have some bearing on the overall performance
of the composite membranes. The membranes according to the
invention can be prepared for instance, by coating a membrane with
a substance containing the material of the coating such that the
coating has a resistance to fluid flow which is low in comparison
to the total resistance of the multicomponent membrane. The coating
may be applied in any suitable manner; e.g., by a coating operation
such as spraying, brushing, immersion in an essentially liquid
substance comprising the material of the coating or the like. The
material of the coating is preferably contained in an essentially
liquid substance when applied and may be in a dispersion or
solution using a dispersion or solvent for the material of the
coating which is substantially a nonsolvent for the material of the
membrane. Advantageously, the substance containing the material of
the coating is applied to one surface of the separation membrane,
and the other side of the separation membrane is subjected to a
lower absolute pressure. However, the invention itself is not
limited by the particular method by which the material of the
coating is applied.
Particularly advantageous materials for the coating have relatively
high permeability constants for fluids such that the presence of a
coating does not unduly reduce the permeation rate of the
multicomponent membrane for desired components. The added
resistance to fluid flow of the coating is preferably relatively
small in comparison to the resistance of the membrane but high with
respect to water or other condensible agents or impurities.
Based on estimates of the average pore diameter of the membrane,
materials for the coating of appropriate molecular size can be
chosen. If the molecular size of the material of the coating is too
large to be accommodated by the pores of the membrane, the material
may not be useful. If, on the other hand, the molecular size of the
material for the coating is too small, it may be drawn through the
pores of the membrane during coating and/or separation operations.
Thus, with membranes having larger pores, it may be desirable to
employ materials for coating having larger molecular sizes. When
the pores are in a wide variety of sizes, it may be desirable to
employ a polymerizable material for the coating material which is
polymerized after application to the membrane, or to employ two or
more coating materials of different molecular sizes; e.g., by
applying the materials of the coating in order of their increasing
molecular sizes.
Coated polymer membranes were evaluated for resistance to the
effects of water in the fluids to be separated. Coated carbon
membranes were evaluated at humidity levels ranging from 23% to 85%
relative humidity. The performance of the membranes is
characterized in terms of membrane productivity and selectivity.
The Examples demonstrate a correlation between the level of
relative humidity and the loss in membrane performance, with the
most severe occurring at the highest relative humidity levels. At
relatively low relative humidity levels, the coated carbon
membranes retain the pre-exposure selectivity and a significant
degree of the pre-exposure productivity. Thus, it is believed the
coating ameliorates the humidity effects on the membranes. By
depositing a layer of the coating on the membrane, it is believed
that the thermodynamic activity of water at the surface of the
membrane is reduced by reducing the flux of water relative to that
of the fluid to be separated through the coating.
This advantageously enables coated membranes to be exposed to
higher levels of relative humidity or other condensible agents or
impurities while maintaining the performance levels seen at lower
levels of relative humidity or other condensible agents or
impurities levels.
The pressure normalized flux of gases through membranes can be
defined as: ##EQU1## wherein cm.sup.3 (STP)/sec is the flux (flow
rate) in units volume per seconds of permeated gas at standard
temperatures and pressure, cm.sup.2 is the area of film, and cm Hg
is the pressure (or driving force).
The selectivity of a membrane in separating a 50/50 mixture of a
two-component fluid is defined as the ratio of the rate of passage
of the more readily passed component to the rate of passage of the
less readily passed component. Selectivity may be obtained directly
by contacting a membrane with a known mixture of fluids and
analyzing the permeate. Alternatively, a first approximation of the
selectivity is obtained by calculating the ratio of the rates of
passage of the two components determined separately on the same
membrane under equivalent driving pressure differences. Rates of
passage may be expressed in GPU units. As an example of
selectivity, a O.sub.2 /N.sub.2 =10 indicates that the subject
membrane allows oxygen gas to pass through at a rate 10 times that
of nitrogen for a 50/50 feed mixture.
Relative Humidity is defined as the ratio of the partial pressure
of the water vapor to the vapor pressure of the liquid at a given
temperature.
The invention as described herein is useful for the separation of,
for example, oxygen from nitrogen; hydrogen from at least one of
carbon monoxide, carbon dioxide, helium, nitrogen, oxygen, argon,
hydrogen sulfide, nitrous oxide, ammonia, and hydrocarbon of 1 to
about 5 carbon atoms, especially methane, ethane and ethylene;
ammonia from at least one of hydrogen, nitrogen, argon, and
hydrocarbon of 1 to about 5 carbon atoms, e.g., methane; carbon
dioxide from at least one of carbon monoxide and hydrocarbon of 1
to about 5 carbon atoms, e.g., methane; hydrogen sulfide from
hydrocarbon of 1 to about 5 carbon atoms, for instance, methane,
ethane, or ethylene; and carbon monoxide from at least one of
hydrogen, helium, nitrogen, and hydrocarbon of 1 to about 5 carbon
atoms. It is emphasized that the invention is also useful for
liquid separations and is not restricted to these particular
separation applications or gases nor the specific multicomponent
membranes in the examples.
EXAMPLES
The invention will now be further illustrated by way of the
following Examples, which are considered to be illustrative only,
and nonlimiting. The coatings and types of membranes examined are
described in the Glossary. The process for formation, coating and
testing of the membranes is described below:
A. Formation of Carbon Membranes
The carbon membranes were produced by pyrolyzing hollow fiber
polymeric materials in a tube furnace as follows:
Individual fibers (8-10 inches long) are placed on piece of
stainless steel mesh (about 1.5-2".times.12") and held in place by
wrapping a length of bus wire around the mesh and fibers. The mesh
support and fibers are then placed in a quartz tube of 2" diameter
which sits in a 24" Thermocraft tube furnace. The tube is centered
so that the entire fiber length is within the effective heating
zone. A pinch of polysulfone (about 0.04 gm) is spread along a 4"
strip of aluminum foil and placed beneath the mesh support. The
system is evacuated until the pressure is 0.15 mmHg or lower as
determined by a mercury manometer. At this point liquid nitrogen is
added to a cold trap to prevent back diffusion of oil vapors from
the vacuum pump, and then the heating cycle is initiated.
Two different heating protocols are used depending on whether a
tighter or more open pore network is desired. The tighter pore
network is produced with the higher temperature (550.degree. C.)
protocol. The temperature is controlled by an Omega temperature
controller. The two heating profiles are as follows:
______________________________________ 500.degree. C. protocol
550.degree. C. protocol ______________________________________
SP.sub.0 - 50.degree. C. SP.sub.0 - 50.degree. C. T.sub.1 - 0:15
T.sub.1 - 0:15 SP.sub.1 - 250.degree. C. SP.sub.1 - 250.degree. C.
T.sub.2 - 1:00 T.sub.2 - 1:14 SP.sub.2 - 485.degree. C. SP.sub.2 -
535.degree. C. T.sub.3 - 1:00 T.sub.3 - 1:00 SP.sub.3 - 500.degree.
C. SP.sub.3 - 550.degree. C. T.sub.4 - 2:00 T.sub.4 - 2:00 SP.sub.4
- 500.degree. C. SP.sub.4 - 550.degree. C.
______________________________________ SP = set point T = time
(hrs:min)
After the heating cycle is complete, the system is allowed to cool
under vacuum. The carbon membranes are not removed from the furnace
until the system temperature drops below 40.degree. C.
B. Single Fiber Module Construction.
The characterization work was performed with single fiber test
modules shown in FIG. 1. The module is constructed from 1/4"
stainless steel tubing 12 and Swagelok 1/4" tees 14. A small length
of tubing 12 is attached to each arm of the tee 14 to form a
housing as shown in FIG. 1A. The test module housing 10 has tubing
12 which has its ends plugged with epoxy 18. The hollow fiber
carbon membrane 16 is threaded through the housing 10 so that a
length of carbon fiber extends on each end. Five minute epoxy is
used to plug the ends of the tubing 18, as shown in FIG. 1B and the
ends of the carbon membrane are snapped off after the epoxy hardens
as shown in FIG. 1C.
C. Coating the Membrane
The coating process takes place after the carbon membrane has been
mounted in a module. Coating solutions were made by dissolving the
appropriate polymeric material in the appropriate solvent so that
the polymer concentration is generally in the 0.5 to 1.5% range.
Because bore side feed method of operation was used our test
system, the coating was applied to the bore side of the hollow
fiber membrane. The solution was introduced at one end of the
hollow fiber membrane, where it flowed down the length of the fiber
and out the other end. A helium pressure head was used to force the
flow and while the amount of coating solution fed through varied,
it was generally in the range of 0.5 to 1.0 cc. During this
process, vacuum was being pulled on the shell side of the membrane.
This procedure provided enough coating solution on the membrane
wall to make a layer in the 0.5-5.0 .mu.m thickness range when the
solvent evaporated. Dry air was fed through the bore until solvent
removal is complete.
D. Membrane Test System
A diagram of the membrane test system is shown in FIG. 2. The
membrane module was attached in a bore feed method of operation and
feed was supplied from compressed gas cylinders. The feed gas could
either be used dry or passed through a humidity chamber prior to
the membrane module. The humidity chamber consisted of a stainless
steel cannister in which different saturated salt solutions were
used to control the relative humidity level. The relative humidity
was also independently verified with an RH meter at an exit
port.
Permeate from the shell side of the hollow fiber membrane was
pulled by vacuum first through a sample volume and then through a
gas chromatograph ("GC") sample loop. The sample volume was
connected to a Baratron pressure transducer and total flux
measurements were made by closing the valve to vacuum and measuring
the pressure increase with time. Composition of the permeate was
determined by GC and then the flux of each individual species was
calculated.
E. Glossary
Coating A means TEFLON.RTM. AF 1600 (commercially available from E.
I. du Pont de Nemours and Company) which is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and tetrafluoroethylene having a
T.sub.g of about 160.degree. C.
Coating B means TEFLON.RTM. AF 2400 (commercially available from E.
I. du Pont de Nemours and Company which is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and tetrafluoroethylene having a
T.sub.g of about 240.degree. C.
Carbon membrane (1) means a membrane fabricated from a polyimide
which is derived from a reaction of 2,4,6-trimethyl-1,3-phenylene
diamine,
5,5-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandion
and 3,3',4,4'-biphenyl tetra carboyxlic acid dianhydride, pyrolyzed
according to the 500.degree. pyrolysis procedure described above.
The membrane is commercially available from E. I. du Pont de
Nemours and Company. Carbon membrane (2) means a membrane described
above pyrolyzed according to the 550.degree. pryolysis procedure
described above.
Tables 1 and 2 compare the performance of coated and uncoated
carbon membranes, respectively, which are exposed to humidity for
18-52 hours. After exposure to 62-65% Relative Humidity, the coated
membranes have a negligible change in O.sub.2 /N.sub.2 selectivity
and a moderate loss in productivity. After exposure to 44-70%
Relative Humidity, the uncoated membranes have a negligible change
in O.sub.2 /N.sub.2 selectivity with a more significant loss in
productivity.
Tables 3 and 4 compare the performance of coated and uncoated
carbon membranes, respectively, which are exposed to 83-85%
Relative Humidity for 24 hours. The coated membranes have a
negligible change or an increase in O.sub.2 /N.sub.2 selectivity
with a small to moderate loss in productivity. The uncoated
membranes exhibit a much greater loss in productivity.
Table 5 compares the performance of coated and uncoated carbon
membranes which are exposed to 83-85% Relative Humidity for 22-24
hours. The coated membrane shows a slight loss in H.sub.2 /CH.sub.4
selectivity with a small loss in productivity. The uncoated
membrane exhibits a very large loss in selectivity and a very large
loss in productivity.
TABLE 1
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Composite Membrane Performance at 62-65% Relative Humidity
Percentage Pre- Percentage Pre- Post- Change in Relative exposure
Post- Change in exposure exposure Selectivity Membrane Humidity
O.sub.2 flux exposure O.sub.2 Flux After O.sub.2 /N.sub.2 O.sub.2
/N.sub.2 after Exam. Composition Exposure (GPU) Flux (GPU) Exposure
Selectivity Selectivity Exposure
__________________________________________________________________________
1 Carbon membrane 62-65% 23.9 13.6 -43% 7.3 7.4 +1% (1) coated with
for 18 0.45 .mu.m poly(4- hours methyl-1-pentene) 2 Carbon membrane
62-65% 34.5 21.4 -38% 9.6 9.8 +2% (1) coated with for 24 0.9 .mu.m
coating A hours 3 Carbon membrane 62-65% 33.5 26.8 -20% 8.3 8.8 +6%
(1) coated with for 52 4 .mu.m coating B hours
__________________________________________________________________________
TABLE 2
__________________________________________________________________________
Uncoated Membrane Performance Percentage Pre- Percentage Pre- Post-
Change in Relative exposure Post- Change in exposure exposure
Selectivity Comp Membrane Humidity O.sub.2 flux exposure O.sub.2
Flux After O.sub.2 /N.sub.2 O.sub.2 /N.sub.2 after Exam.
Composition Exposure (GPU) Flux (GPU) Exposure Selectivity
Selectivity Exposure
__________________________________________________________________________
4 Carbon membrane 44% for 25.2 12.9 -49% 9.6 9.6 0% (1) with no
coating 19 hours 5 Carbon membrane 67-70% 25.5 11.2 -56% 9.3 8.9
-4.3% (1) with no coating for 17 hours
__________________________________________________________________________
TABLE 3
__________________________________________________________________________
Composite Membrane Performance at 83-85% Relative Humidity
Percentage Pre- Percentage Pre- Post- Change in Relative exposure
Post- Change in exposure exposure Selectivity Membrane Humidity
O.sub.2 flux exposure O.sub.2 Flux After O.sub.2 /N.sub.2 O.sub.2
/N.sub.2 after Exam. Composition Exposure (GPU) Flux (GPU) Exposure
Selectivity Selectivity Exposure
__________________________________________________________________________
6 Carbon membrane 83-85% 29.9 26.6 -11% 7.8 7.7 -1% (1) coated with
for 24 3.5 .mu.m coating A hours 7 Carbon membrane 83-85% 32.3 19.1
-41% 8.4 9.6 +14% (1) coated with for 24 4 .mu.m coating B hours 8
Carbon membrane 83-85% 27.8 15.6 -44% 11.8 12.7 +8% (2) coated with
for 24 2.3 .mu.m coating B hours
__________________________________________________________________________
TABLE 4
__________________________________________________________________________
Uncoated Membrane Performance at 83-85% Relative Humidity
Percentage Pre- Percentage Pre- Post- Change in Relative exposure
Post- Change in exposure exposure Selectivity Comp Membrane
Humidity O.sub.2 flux exposure O.sub.2 Flux After O.sub.2 /N.sub.2
O.sub.2 /N.sub.2 after Exam. Composition Exposure (GPU) Flux (GPU)
Exposure Selectivity Selectivity Exposure
__________________________________________________________________________
9 Carbon membrane 83-85% 30.7 12.6 -59% 10.5 10.7 +2% (1) with no
coating for 24 hours 10 Carbon membrane 83-85% 31.1 15.6 -50% 13.4
12.8 -5% (2) with no coating for 24 hours
__________________________________________________________________________
TABLE 5
__________________________________________________________________________
Coated and Uncoated Membrane Performance at 83-85% Relative
Humidity for H.sub.2 /CH.sub.4 Separations Percentage Pre-
Percentage Pre- Post- Change in Relative exposure Post- Change in
exposure exposure Selectivity Membrane Humidity O.sub.2 flux
exposure O.sub.2 Flux After H.sub.2 /CH.sub.4 H.sub.2 /CH.sub.4
after Exam. Composition Exposure (GPU) Flux (GPU) Exposure
Selectivity Selectivity Exposure
__________________________________________________________________________
Exam. Carbon 83-85% 113.8 97.9 -14% 206 196 -5% 11 membrane (2) for
24 coated with hours 2.8 .mu.m Teflon AF1600 Comp Carbon 83-85%
97.8 12.7 -87% 520 106 -80% Exam membrane (2) for 22 12 with no
coating hours
__________________________________________________________________________
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